The present invention relates to a power supply circuit for generating potentials required for driving a liquid crystal, and to a liquid crystal device and an electronic device using same.
FIG. 20 is the configuration of a conventional power supply circuit for generating potentials required for driving a liquid crystal by resistance division. The first to fifth resistors R1 to R5 are connected in series across a first potential-supply line 401 supplying a high potential V0 and a second potential-supply line 402 supplying a low potential V5. Potentials V1 to V4 between V0 and V5 are generated by dividing the potential difference (V0-V5) between the first and second potential-supply lines by resistors R1 to R5.
These potentials V0 to V5 are used as the potentials of common signals COM0, COM1, COM2, and so on applied to common electrodes that are scanning electrodes and of segment signals SEGn applied to segment electrodes that are signal electrodes, as shown in FIG. 16. In the example shown in FIG. 20, potentials V0 and V5 become select potentials of common signals, and potentials V1 and V4 become non-select potentials of common signals. Potentials V0 and V5 become, for example, on-potentials of segment signals, and potentials V2 and V3 become, for example, off-potentials of segment signals.
When potentials V1 to V4 are generated by resistor division as shown in FIG. 20, the current driving capability of a power supply circuit is dependent on the values of the resistors used for dividing voltage. Although a power supply circuit for driving a liquid crystal needs a current driving capability according to the load (liquid crystal) driven by it, the current driving capability of a power supply circuit is limited by the resistors used. In particularly, when the values of the resistors are large and the load of the crystal to be driven is large, the potentials generated by resistor division vary beyond permissible limits. As a result, the liquid crystal display device does not produce a normal display. For a liquid crystal display device to make normal display even in the case where the load to drive the liquid crystal display is large, the current driving capability of a power supply circuit must be increased. This requires the values of the resistors to be decreased. However, decreasing the values of the resistors for resistor increases the power consumption in the power supply circuit.
FIG. 21 is the circuit diagram of another conventional power supply circuit for driving a liquid crystal device, and differs from the power supply circuit of FIG. 20 in that voltage-follower operational amplifiers 403 to 406 are respectively connected to the output lines of potentials V1 to V4. The voltage-follower operational amplifiers 403 to 406 perform impedance conversion and output of the input potentials V1 to V4.
Although the circuit of FIG. 21 can decrease the power consumption by the resistors for resistor division, this circuit requires four voltage-follower operational amplifiers 403 to 406. Furthermore, this operational amplifier has a large power consumption because of requirement of a specific circuit configuration such as differential pair or the like.
An object of the present invention is therefore to provide a power supply circuit for driving a liquid crystal which can decrease the power consumption, and a liquid crystal device and an electronic device using same.
A first aspect of the present invention provides a power supply circuit for generating potentials used to drive a liquid crystal, the power supply circuit comprising:
first to fourth switches connected in series between a high potential line and a low potential line;
a switch drive circuit which drives the first to fourth switches so that the period of time in which the first and third switches are on and the period of time in which the second and fourth switches are on are alternate; and
a plurality of capacitors of which connection state is switched alternately between series and parallel connections by a switching operation of the switch drive circuit,.
wherein a potential between the second and third switches converges a middle potential between potentials of the high and low potential lines by a switching operation of the switch drive circuit.
According to this aspect of the present invention, the amount of electric charge stored in the plurality of capacitors becomes stabilized because of the switching operation described above. Consequently, the potential between the second and third switches converges the middle potential between the potential difference of the high and low potential lines.
Since no current flows through the circuit when the amount of electric charge stored in the capacitors becomes stabilized, the power consumption can be decreased. In addition, because the potentials become stabilized without being affected by the variation in the capacitances of the plurality of capacitors, an accurate potential can be generated.
When first to third midpoints are midpoints of switch-intervals formed by being divided by the first to fourth switches, the power supply circuit may comprise:
a first capacitor connected between the high potential line and the second midpoint;
a second capacitor connected between the second midpoint and the low potential line; and
a third capacitor connected between the first midpoint and the third midpoint.
By connecting the three capacitors in this manner, the connection of the third capacitor to the first and second capacitors is alternately switched between series and parallel connections by the above-described switching operation.
In this configuration, the first and second capacitors may be replaced by capacitors of a liquid crystal layer formed by supplying potentials of the high and low potential lines and the second midpoint to the liquid crystal layer.
The plurality of capacitors may also be formed of a first capacitor connected between the high potential line and the second midpoint; and a second capacitor connected between the first midpoint and the third midpoint. Further, the plurality of capacitors may also be formed of a first capacitor connected between the second midpoint and the low potential line; and a second capacitor connected between the first midpoint and the third midpoint.
In either configuration, the connection of the first and second capacitors is switched alternately between series connection and parallel connection.
Another aspect of the present invention provides a power supply circuit for generating potentials used to drive a liquid crystal, the power supply circuit comprising: a main power supply circuit generating a potential between potentials of a first potential-supply line and a second potential-supply line; a first sub-power supply circuit generating a potential between potentials of the first potential-supply line and an output line of the main power supply circuit; and a second sub-power supply circuit generating a potential between potentials of the output line of the main power supply circuit and the second potential-supply line. The power supply circuit described above may be used for at least one of the main power supply circuit and the first and second sub-power supply circuits.
By using the power supply circuit described above for all of the main power supply circuit and the first and second sub-power supply circuits, five-level liquid crystal drive potentials V0 to V4 used for a xc2xc bias driving method can be accurately generated.
To generate liquid crystal drive potentials used for a bias driving method of xc2xc or less, for example, six-level potentials V0 to V5, it is preferable to use a resistor division method for the main power supply circuit for generating two-level potentials V2 and V3 between the high potential V0 and the low potential V5 and use the potentials V2 and V3 impedance-convert through impedance-conversion circuits (formed of an operational amplifier, for example). In this case, the first sub-power supply circuit generates a potential V1 between the potentials V0 and V2, and the second sub-power supply circuit generates a potential V4 between the potentials V3 and V5.
By this configuration, compare to a conventional power supply circuit which needs four operational amplifiers to generates a potential of liquid crystal, present invention can omit two operational amplifiers. As a result, the manufacturing cost can be decreased because of the reduced chip size. Electric power consumption may also be decreased.
P-type MOS transistors can be used for a first to fourth switches (sub-switches) in the second sub-power supply circuit. In addition, N-type MOS transistors can be used for a fifth to eighth switches (sub-switches) in the second sub-power supply circuit.
The switching operation described above is made possible by applying the high potential V0 and the low potential V5 (both potentials are the select potential of the scanning signal) alternately to the gate of the P-type MOS and N-type MOS transistors.
Since this configuration makes it possible to apply a greater voltage between the source and gate, transistors of the same performance can be made in a smaller size. Consequently, the manufacturing cost of the power supply circuit can be decreased because of the reduced chip size.
A liquid crystal device of the present invention and an electronic device having the liquid crystal device of the present invention include the power supply circuit for a liquid crystal described above. Since the power supply circuit of the present invention can reduce the power consumption of the liquid crystal device, it is particularly useful for portable electronic devices.